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$minN (aggregation accumulator)

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  • Definition
  • Syntax
  • Behavior
  • Restrictions
  • Examples
$minN

New in version 5.2.

Returns an aggregation of the minimum value n elements within a group. If the group contains fewer than n elements, $minN returns all elements in the group.

{
$minN:
{
input: <expression>,
n: <expression>
}
}
  • input specifies an expression that is the input to $minN. It is evaluated for each element in the group and $minN preserves the minimum n values.

  • n limits the number of results per group and n has to be a positive integral expression that is either a constant or depends on the _id value for $group.

$minN compares input data following the BSON Comparison order to determine the appropriate output type. When the input data contains multiple data types, the $minN output type is the lowest in the comparison order.

  • $minN filters out null and missing values.

Consider the following aggregation that returns the minimum n documents from a group:

db.aggregate( [
{
$documents: [
{ playerId: "PlayerA", gameId: "G1", score: 1 },
{ playerId: "PlayerB", gameId: "G1", score: 2 },
{ playerId: "PlayerC", gameId: "G1", score: 3 },
{ playerId: "PlayerD", gameId: "G1" },
{ playerId: "PlayerE", gameId: "G1", score: null }
]
},
{
$group:
{
_id: "$gameId",
minimumThreeScores:
{
$minN:
{
input: "$score",
n: 4
}
}
}
}
] )

In this example:

  • $documents creates the literal documents that contain player scores.

  • $group groups the documents by gameId. This example has only one gameId, G1.

  • PlayerD has a missing score and PlayerE has a null score. These values are both ignored.

  • The minimumThreeScores field is specified as $minN with input : "$score" and returned as an array.

  • Since there are only 3 documents with scores minN returns the minimum 3 score fields even though n = 4.

[
{
_id: 'G1',
minimumThreeScores: [ 1, 2, 3 ]
}
]

Both $minN and $bottomN accumulators can accomplish similar results.

In general:

  • $minN has the advantage of finding minimum values in no particular sort order. If you want to know the minimum values for n documents use $minN.

  • If guaranteing a particular sort order is a requirement use $bottomN.

  • Use $bottomN if you don't intend on sorting on the output values.

You can use $minN as an accumulator.

$minN is supported as an aggregation expression.

$minN is supported as a window operator.

Aggregation pipelines which call $minN are subject to the 100 MB limit. If this limit is exceeded for an individual group, the aggregation fails with an error.

Consider a gamescores collection with the following documents:

db.gamescores.insertMany([
{ playerId: "PlayerA", gameId: "G1", score: 31 },
{ playerId: "PlayerB", gameId: "G1", score: 33 },
{ playerId: "PlayerC", gameId: "G1", score: 99 },
{ playerId: "PlayerD", gameId: "G1", score: 1 },
{ playerId: "PlayerA", gameId: "G2", score: 10 },
{ playerId: "PlayerB", gameId: "G2", score: 14 },
{ playerId: "PlayerC", gameId: "G2", score: 66 },
{ playerId: "PlayerD", gameId: "G2", score: 80 }
])

You can use the $minN accumulator to find the minimum three scores in a single game.

db.gamescores.aggregate( [
{
$match : { gameId : "G1" }
},
{
$group:
{
_id: "$gameId",
minScores:
{
$minN:
{
input: ["$score","$playerId"],
n:3
}
}
}
}
] )

The example pipeline:

  • Uses $match to filter the results on a single gameId. In this case, G1.

  • Uses $group to group the results by gameId. In this case, G1.

  • Specifies the fields that are input for $minN with input : ["$score","$playerId"].

  • Uses $minN to return the first three score elements for the G1 game with n : 3.

The operation returns the following results:

[
{
_id: 'G1',
minScores: [ [ 1, 'PlayerD' ], [ 31, 'PlayerA' ], [ 33, 'PlayerB' ] ]
}
]

You can use the $minN accumulator to find the minimum n scores in each game.

db.gamescores.aggregate( [
{
$group:
{
_id: "$gameId",
minScores:
{
$minN:
{
input: ["$score","$playerId"],
n: 3
}
}
}
}
] )

The example pipeline:

  • Uses $group to group the results by gameId.

  • Uses $minN to return the minimum three score elements for each game with n: 3.

  • Specifies the fields that are input for $minN with input: ["$score","$playerId"].

The operation returns the following results:

[
{
_id: 'G2',
minScores: [ [ 10, 'PlayerA' ], [ 14, 'PlayerB' ], [ 66, 'PlayerC' ] ]
},
{
_id: 'G1',
minScores: [ [ 1, 'PlayerD' ], [ 31, 'PlayerA' ], [ 33, 'PlayerB' ] ]
}
]

You can also assign the value of n dynamically. In this example, the $cond expression is used on the gameId field.

db.gamescores.aggregate([
{
$group:
{
_id: {"gameId": "$gameId"},
gamescores:
{
$minN:
{
input: ["$score","$playerId"],
n: { $cond: { if: {$eq: ["$gameId","G2"] }, then: 1, else: 3 } }
}
}
}
}
] )

The example pipeline:

  • Uses $group to group the results by gameId.

  • Specifies the fields that input for $minN with input : ["$score","$playerId"].

  • If the gameId is G2 then n is 1, otherwise n is 3.

The operation returns the following results:

[
{ _id: { gameId: 'G2' }, gamescores: [ [ 10, 'PlayerA' ] ] },
{
_id: { gameId: 'G1' },
gamescores: [ [ 1, 'PlayerD' ], [ 31, 'PlayerA' ], [ 33, 'PlayerB' ] ]
}
]

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